Ongoing toilet issue and needing another opinion

Our toilet has ongoing issues for about 2 months now. We’ve had plumbers and cameras out multiple times and we’ve yet to determine issue. Wondering if anyone has run into this or has other ideas.

Mid-October our toilet clogged. I plunged it and it backed up into the tub. Knowing this was outside my tools and knowledge, I brought in the plumber. They pulled toilet, found a clog, pushed it out, and said as they pulled the tool back it felt like it snagged on something so they brought a camera in. Camera found no issue. New phalange was put on since ours was broken and toilet reset. The phalange isn’t a traditional one, they did a newer style that is put into the floor opening.

Shortly after this I noticed the toilet would flush, fill, and then water would drop about half inch to inch almost right away. Don’t know if it was like this right after the reset bc I wasn’t paying attention but did discover it a few weeks later. This time they went from the clean out valve and did the camera out a 100 ft which would take us off property. No bellies, etc that they could find. Siphon issue remained and this company said maybe a vent issue but they don’t get on roofs.

Toilet backed up yesterday. Bowl started to overflow so we turned off water and brought plumber back. They tried to auger it and plunge it. Plunging it caused backup into tub again. Auger didn’t help - it was too short. They said they’ll need to auger with a bigger tool later this week, maybe another camera. They won’t rule out vent issue but they said those are rare. Now they are thinking the pipe has sunk and is no longer pitched properly. Said maybe a high pressure toilet could help or we may have to lift the sagging plumbing.

More info that might help: This plumbing is on a slab.

Vent is only for this bathroom. Kitchen and 2nd bath have their own vents and no issues outside of that toilet being super old and the flush is weak - the hole that pushes the water into tank to flush is crusty so it’s prob just not a good push. The outgoing waste pipes don’t connect to the 1st bathroom pipes until after my basements floor drain - no water has come up that drain.

This pipe did have a crack about 2 years ago but it was fixed and one of the camera jobs this time looked at it and said they saw the fix and it was still fixed.

Toilet in this bathroom is 5ish years old. House 40 years old.

City came out and confirmed there is no blockage or problems on their end and they showed me the photos.

Anyone have any other ideas or things we haven’t considered yet?